1. Field of the Invention
The present invention relates to an inkjet recording apparatus and a recording method, and more particularly, to an inkjet recording apparatus and an inkjet recording method whereby continuous circulation of liquid can be achieved while maintaining the back pressure in a recording head of an inkjet type.
2. Description of the Related Art
The inkjet recording apparatus comprises a recording head of an inkjet type (an inkjet head, hereinafter simply called “head”) which has a plurality of nozzles, and records an image by ejecting ink droplets respectively from the nozzles. Such apparatuses are used widely from commercial to industrial applications, due to their low operating noise, low running costs, and their capacity to record high quality images onto recording media of many various types. The ink ejection method used in the recording head may be a piezoelectric method in which ink droplets are ejected from nozzles by utilizing the displacement of piezoelectric elements to pressurize the ink inside pressure chambers, or a thermal method in which ink droplets are ejected from nozzles due to the pressure created by the growth of gas bubbles which are generated inside pressure chambers by means of the thermal energy created by heating elements, such as heaters, or the like.
In an inkjet recording apparatus, it is common to use a method which supplies ink to the recording head from a main tank and via sub tanks. According to a sub tank supply method of this kind, it is possible to suppress the internal pressure variation of the recording head, and it is also possible to improve the ejection stability of the recording head.
On the other hand, if air bubbles enter into the recording head or the ink inside the recording head (and in particular, in the vicinity of the nozzles) increases in viscosity, then there is a possibility that these factors can give rise to deterioration of the image quality, since fluctuation occurs in the droplet amount (volume) and ejection direction (direction of flight) of the ink droplets which are ejected from the respective nozzles, and nozzles suffering ejection failure occur due to blockages, and so on. In order to resolve problems of this kind, various technologies which circulate the ink inside the recording head have been proposed (see, for example, Japanese Patent Application Publication No. 2000-280493 and Japanese Patent Application Publication No. 10-114081).
Japanese Patent Application Publication No. 2000-280493 discloses technology of a system comprising a reserve tank which is provided in a unified fashion with the recording head and two sub tanks (a supply sub tank and an expulsion sub tank) which are connected to the reserve tank, and the ink in the supply sub tank is circulated to the expulsion sub tank via the reserve tank by reducing the pressure of the expulsion sub tank by means of a pump.
Japanese Patent Application Publication No. 10-114081 discloses technology of a system in which a sub tank is connected to one ink inlet port of two ink inlet ports which are provided in a recording head, and an ink cartridge is connected to the other ink inlet port, an outward and return ink circulation being created between the sub tank and the ink cartridge via the recording head. More specifically, by pressurizing an ink bag (ink supply source) provided inside the ink cartridge (sealed space) by means of an air pump, the ink inside the ink bag flows into the sub tank via the recording head, and when the sub tank has filled with ink, the air pump is halted and the ink inside the sub tank flows in reverse to the ink cartridge via the recording head due to the liquid head differential caused by the height differential between the recording head and the ink cartridge. If the volume inside the sub tank becomes smaller, then the air pump is operated and ink is replenished to the sub tank from the ink cartridge via the recording head. Thereupon, the ink replenishment operation described above is carried out each time the ink of the sub tank flows in reverse and reduces in volume. By means of an outward and return ink circulation of this kind, air bubbles present inside the recording head are eliminated and increase in the viscosity of the ink can be prevented.
However, there are problems of the following kinds associated with the related art ink circulation technology which is described above.
In the invention described in Japanese Patent Application Publication No. 2000-280493, by sealing and reducing the pressure of the expulsion sub tank of two sub tanks (a supply sub tank and the expulsion sub tank) which are connected to a reserve tank that is provided in a unified fashion with the recording head, a circulation operation is carried out which simply moves the ink forcibly from the supply sub tank to the expulsion sub tank via the reserve tank, the volume of ink which can be circulated in this way being dependent on the remaining amount of ink in the sub tank, and it is not possible to circulate ink continuously during printing, and the like. Furthermore, since the ink supply system uses the liquid head differential, then it is necessary to dispose the two sub tanks below the recording head, and therefore the flow channels between the recording head and the sub tanks become long and there is a large variation in the back pressure variation as a result of pressure loss. Consequently, there is a problem in that the ejected droplet volume varies and the print quality declines thereby.
The invention described in Japanese Patent Application Publication No. 10-114081 simply moves the ink in the outward and return directions via the recording head between one tank (an ink cartridge) and another tank (sub tank), and it is not able to circulate the ink continuously in one direction at all times (for example, the direction from the ink cartridge to the sub tank). Consequently, when the direction of movement of the ink (direction of ink circulation) is switched in accordance with the remaining amount of ink in the sub tank, then the back pressure of the recording head is liable to vary due to the pressure change which occurs during switching. Furthermore, since the sub tank uses a hydrostatic pressure system, then in the case of a recording head where the direction of ejection lies in the horizontal direction, it is possible to dispose the sub tank in the vicinity of the recording head and vertically below the recording head, but when applied to a recording head in which the direction of ejection is vertically downwards, then due to the position of the sub tank, the flow channel between the sub tank and the recording head becomes long and the loss of pressure becomes large. Consequently, there is a problem in that the ejected droplet volume varies and the print quality declines.